Method and a control unit for starting a compressor

ABSTRACT

A method and a control unit for starting a compressor ( 3 ) in a refrigeration system ( 1 ) further comprising a condenser ( 2 ) and an evaporator ( 4 ). If an attempt to start-up the compressor ( 3 ) is unsuccessful a condenser cooling means, e.g. a fan, is started in order to provide cooling for the condenser ( 2 ). Thereby, if the temperature of the condenser is sufficiently high to cause an increased pressure on the condenser side of the compressor ( 3 ), cooling is provided for the condenser ( 2 ) in order to ensure a reliable start-up of the compressor ( 3 ). On the other hand, if the temperature of the condenser ( 2 ) is sufficiently low to ensure a reliable start-up of the compressor ( 3 ), cooling is not provided, thereby saving power while ensuring reliable start-up of the compressor ( 3 ).

CROSS REFERENCE TO RELATED APPLICATIONS:

Applicant hereby claims foreign priority benefits under U.S.C. §119 from Danish Patent Application No. PA 2006 00052 filed on Jan. 12, 2006, the contents of which are incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to a method for starting a compressor in a refrigeration system in a manner which conserves power while ensuring reliable start-up of the compressor. Furthermore, the present invention relates to a control unit for controlling the start-up of a compressor and a starter assembly comprising such a control unit.

BACKGROUND OF THE INVENTION

In some refrigeration applications the refrigeration system will be subject to relatively large variations in the ambient temperature, resulting in relatively large temperature variations of the various parts of the refrigeration system. This is in particular the case in automotive applications, such as refrigeration systems positioned in vehicles, e.g. cars, trucks, campers, waterborne vehicles, such as boats, airplanes, etc. Increasing temperatures of the parts of the refrigeration system will lead to increasing pressure of the refrigerant. This increase in temperature, in particular when it occurs on the condenser side of the compressor, has the consequence that it is difficult to restart the compressor following a period in time where the compressor has been stopped.

It has previously been attempted to address this problem by providing cooling to the condenser by means of a rotating fan before it is attempted to start the compressor. This is described in U.S. Pat. No. 4,941,325 and in U.S. Pat. No. 6,155,062.

U.S. Pat. No. 4,941,325 describes a method and apparatus or control system for electronically sequencing the main components in central air-conditioning and heat pump systems. When comfort production or conditioned air is needed, the outside fan motor is initially turned on. After a predetermined programmable period of time, the compressor is turned on, and then, after another predetermined programmable period of time, the inside blower is turned on to deliver production air to the space or area being serviced. Thereby the condenser is pre-cooled before the compressor is turned on.

U.S. Pat. No. 6,155,062 describes a mode of operation of an air-conditioning installation of a compression cycle type helicopter cabin or cockpit air conditioning installation. The installation includes a start-up sequence controlled by an inverter controlling the motor-compressor associated with a condenser equipped with a fan and during which the fan of the condenser is first powered up, after which the motor-compressor is powered up to start it at a low speed, after which the motor-compressor is operated under its normal conditions as a function of the cooling requirements of the installation.

A disadvantage of the methods described in U.S. Pat. No. 4,941,325 and U.S. Pat. No. 6,155,062 is that the condenser fan is started in order to provide cooling for the condenser each time an attempt to start the compressor is initiated. Thereby reliable start-up of the compressor is obtained, but at the expense of a higher power consumption. This is particularly disadvantageous in situations where the available power is limited, e.g. when the compressor is powered by a battery.

SUMMARY OF THE INVENTION

It is, thus, an object of the invention to provide a method for starting a compressor in a manner which ensures reliable start-up of the compressor while conserving energy as compared to prior art start-up methods.

It is a further object of the invention to provide a control unit for controlling start-up of a compressor in a reliable and energy conserving manner.

It is an even further object of the invention to provide a starter assembly for starting a compressor in a reliable and energy conserving manner.

According to a first aspect of the invention, the above and other objects are fulfilled by providing a method for starting a compressor in a refrigeration system, the refrigeration system further comprising a condenser and an evaporator, said compressor, condenser and evaporator being interconnected in a circuit in which a refrigerant is allowed to flow, the method comprising the steps of:

-   -   attempting start-up of the compressor, thereby initiating a         start-up sequence,     -   determining whether the attempted start-up of the compressor was         successful,     -   in the case that it is determined that the attempted start-up of         the compressor was unsuccessful, starting a condenser cooling         means, thereby providing cooling for the condenser, and waiting         for a predefined time interval,     -   repeating the above steps until it is determined that the         attempted start-up of the compressor was successful, and     -   ending the start-up sequence.

The refrigeration system may be any suitable kind of refrigeration system operating in a manner which is well known to the skilled person. The method according to the first aspect of the invention is particularly suitable for use in automotive applications, i.e. in refrigeration systems which are movable, e.g. refrigeration systems positioned onboard a vessel, such as a land based vehicle, e.g. a truck, a car, a camper van, etc., a waterborne vessel, such as a boat, a ferry, etc., or an airborne vessel, such as an aircraft, a helicopter, etc.

The step of determining whether the attempted start-up of the compressor was successful may advantageously comprise establishing whether or not a specified rotational speed of the compressor motor has been reached within a specified time interval following the initiation of the attempted start-up of the compressor. This may, e.g., be done using a microcontroller for monitoring and calculating the rotational speed of the compressor. For instance, the attempted start-up may be regarded as ‘successful’ if the compressor is actually started and is running at a rotational speed which is larger than, e.g., 1750 rpm. In some applications the compressor will be damaged due to insufficient lubrication of the compressor if it is running at a rotational speed which is lower than approximately 1750 rpm, and it is therefore important that the attempted start-up of the compressor is aborted if this speed is not reached within a reasonable time.

Alternatively or additionally, the current level supplied to the compressor may be monitored, e.g. by means of a microcontroller. If the current level is relatively high, e.g. above a specified threshold value, this may indicate that the attempted start-up of the compressor is initiated at a too high pressure level, and accordingly it may be determined that the attempted start-up was unsuccessful.

If the attempted start-up of the compressor was unsuccessful, this may be caused by a too high pressure of the refrigerant at the condenser side of the compressor, and this may in turn be caused by a too high surface temperature of the condenser. Therefore a condenser cooling means is started in order to provide cooling for the condenser when this situation occurs. In order to allow the condenser to be cooled sufficiently to ensure a successful start-up of the compressor at the next attempt, a predefined time is allowed to elapse before the next attempted start-up of the compressor is initiated.

The attempted start-ups of the compressor are repeated until an attempt is successful, i.e. until the compressor is running in a normal manner.

According to the method of the first aspect of the invention, the condenser cooling means is only started if it is necessary in order to ensure a successful start-up of the compressor. Thus, when the surface temperature of the condenser is sufficiently low to allow a proper start-up of the compressor, there is no need for cooling the condenser, and the condenser cooling means is accordingly not started. On the other hand, if the surface temperature of the condenser is too high to allow proper start-up of the compressor, it is ensured that the condenser cooling means is started. Thereby power is conserved while at the same time ensuring reliable start-up of the compressor. This is very advantageous.

The predefined time may be within the time interval 10 s to 300 s, such as within the time interval 30 s to 100 s, such as within the time interval 45 s to 80 s, such as approximately 60 s. Alternatively, the predefined time interval may be shorter or longer than the intervals mentions above, depending on the specific application and situation.

In a preferred embodiment the step of starting a condenser cooling means may comprise supplying power to a fan positioned in the vicinity of the condenser, thereby causing the fan to rotate. In this embodiment the condenser cooling means is or comprises a condenser fan which can be switched on or off in order to provide cooling to the condenser when this is necessary.

Alternatively, the condenser cooling means may comprise a heat exchanger. The cooling may in this case be obtained by means of natural heat exchange between tubes and ambient air, e.g. using a so-called ‘wire and tube’ condenser. In this case a fan may additionally be applied in order to increase the cooling capacity. Alternatively, the heat exchanger may be of the so-called ‘fin and tube’ type, possibly with a fan as described above. Alternatively, heat exchange may take place using a tube heat exchanger, e.g. using water as cooling medium. In this case the heat exchanger may be of a recycling type in which the water is recycled, or a non-recycling type in which the water is discarded after use.

The method may further comprise the step of stopping the condenser cooling means after the predefined time has elapsed. According to this embodiment the condenser cooling means is stopped when the attempt to start the compressor is initiated. Alternatively, the condenser cooling means may be allowed to remain in an ‘on’ state after the attempted start-up of the compressor is initiated. The condenser cooling means may even be allowed to continue running for as long as the compressor is running, i.e. the condenser cooling means is stopped when the compressor is once again stopped. In this situation it is ensured that the surface temperature of the condenser does not start increasing undesirably when the compressor is started, and it may even be obtained that the surface temperature of the condenser continues to decrease. Thereby a proper operation of the compressor is ensured.

The method may further comprise the steps of monitoring a temperature in the vicinity of the condenser, and starting the condenser cooling means in case said temperature exceeds a threshold temperature.

The monitored temperature may be a surface temperature of the condenser. Alternatively, it may be an ambient temperature, a temperature in an electronic unit used for controlling the refrigeration system and being positioned in the vicinity of the condenser, or any other suitable temperature being indicative of the surface temperature of the condenser. The idea is that if the temperature at a position near the condenser is high/low, the surface temperature of the condenser will most likely also be high/low. Thus, even if the exact surface temperature of the condenser is not known, knowledge of a temperature at a position in the vicinity of the condenser will give an indication of at least the temperature level of the surface of the condenser. It is sometimes desirable to measure or monitor the temperature in an electronics unit used for controlling the refrigeration system for other purposes, and it may therefore be an advantage to use this temperature for obtaining an indication of the temperature level of the surface of the condenser, since it is measured anyway, and additional method steps or equipment is therefore not required.

In case the monitored temperature is an ambient temperature the threshold temperature could be within the temperature interval 30° C. to 75°, such as within the temperature interval 40° to 60° C., such as approximately 55° C. In case the monitored temperature is a temperature in an electronic unit the threshold temperature could be within the temperature interval 50° C. to 150° C., such as within the temperature interval 70° C. to 130° C., such as within the temperature interval 90° C. to 110° C., such as approximately 100° C. or 105° C. In case the monitored temperature is a surface temperature of the condenser the threshold temperature could be within the temperature interval 50° C. to 110° C., such as within the temperature interval 60° C. to 100° C., such as within the temperature interval 75° C. to 85° C.

When a temperature is monitored as described above, this may be used as a criterion for starting the condenser cooling means when the monitoring reveals that the surface temperature of the condenser is relatively high, regardless of whether or not an attempt to start-up the compressor was successful or unsuccessful. This may, e.g., be the case while the compressor, and thereby the refrigeration system, is running normally, i.e. the compressor has started properly, but the surface temperature of the condenser increases to an unacceptable level while the compressor is running. When the condenser cooling means is switched on under these circumstances, the condenser cooling means is preferably allowed to continue running as long as the compressor continues to run, i.e. until the compressor is stopped.

Alternatively or additionally, the monitoring of the temperature may be used for determining whether or not the condenser cooling means should be started prior to initiating an attempt to start-up the compressor.

According to a second aspect of the invention the above and other objects are fulfilled by providing a control unit for controlling start-up of a compressor in a refrigeration system, the refrigeration system further comprising a condenser and an evaporator, said compressor, condenser and evaporator being interconnected in a circuit in which a refrigerant is allowed to flow, the control unit comprising means for starting a condenser cooling means in case it is determined that an attempted start-up of the compressor was unsuccessful.

It should be noted that a person skilled in the art would readily recognise that any feature described in connection with the first aspect of the invention can also be combined with the second aspect of the invention, and vice versa.

The control unit according to the second aspect of the invention is adapted to perform the method according to the first aspect of the invention, and the remarks set forth above are therefore equally applicable here.

As described above, the condenser cooling means may comprise a fan positioned in the vicinity of the condenser, and the means for starting the condenser cooling means may comprise means for supplying power to the fan, thereby causing it to rotate.

The control unit may further comprise means for monitoring a temperature in the vicinity of the condenser, and the means for starting the condenser cooling means may be adapted to start the condenser cooling means in case said temperature exceeds a threshold temperature. The means for monitoring a temperature may be or comprise a thermometer, a temperature probe, an infrared monitor, and/or any other suitable means for measuring a temperature.

The control unit may preferably form part of a starter assembly for starting a compressor in a refrigeration system, and the starter assembly and/or the compressor may preferably be powered by means of a battery, or by means of another power source in which the available power is limited.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further details with reference to the accompanying drawings in which

FIG. 1 is a diagrammatic view of a refrigeration system comprising a condenser, a compressor, an evaporator and an expansion valve interconnected in a refrigeration circuit,

FIG. 2 is a graph illustrating a method according to an embodiment of the invention, and

FIG. 3 is a flow chart illustrating a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagrammatic view of a refrigeration system 1. The refrigeration system 1 comprises a condenser 2, a compressor 3, an evaporator 4, an expansion valve 5, and a receiver 6, the condenser 2, the compressor 3, the evaporator 4, the expansion valve 5 and the receiver 6 being interconnected in a circuit in which a refrigerant is allowed to circulate when the refrigeration system 1 is operating. The refrigeration system 1 operates in a conventional manner which will not be described here.

In the vicinity of the condenser 2 a condenser fan 7 is positioned. When the condenser fan 7 is rotating cooling is provided for the condenser 2. Similarly, an evaporator fan 8 is positioned in the vicinity of the evaporator 4 for sucking relatively warm air through the evaporator 4, thereby cooling the air, and blowing the cooled air back into the room where the refrigeration system 1 is positioned.

As described above, in case the ambient temperature increases, the pressure of the refrigerant present in the refrigeration system 1 also increases. In particular, the surface temperature of the condenser 2 will increase, and the pressure of the refrigerant present at the condenser side of the compressor 3 will increase. This has the consequence that it becomes difficult to restart the compressor 3 following a period of time where the compressor 3 has been stopped. When this situation occurs, the condenser fan 7 can be switched on, thereby providing cooling for the condenser 2. Thus, when the condenser fan 7 is switched on, the temperature of the condenser 2 decreases, thereby decreasing the pressure of the refrigerant present on the condenser side of the compressor 3. Thereby the start-up of the compressor 3 becomes more reliable. According to the invention the condenser fan 7 is only switched on when it is necessary in order to ensure a reliable start-up of the compressor 3. Thereby energy is conserved as compared to a start-up sequence in which the condenser fan 7 is always switched on prior to an attempt to start the compressor 3. This is particularly an advantage in situations where the available power is limited, e.g. in case the compressor 3 is powered by means of a battery.

FIG. 2 is a graph illustrating a method according to an embodiment of the invention. Compressor state 9, condenser fan state 10 and surface temperature 11 of the condenser are shown as functions of time. In graph 9 a ‘low’ indicates that the compressor is in an ‘off state’, and a ‘high’ indicates that the compressor is in an ‘on state’, i.e. an attempted start-up has been initiated or the compressor is actually running. Similarly, in graph 10 a ‘low’ indicates that the condenser fan is in an ‘off state’, and a ‘high’ indicates that the condenser fan is in an ‘on state’, i.e. the fan is running.

Initially the compressor is ‘off’ and the surface temperature 11 of the condenser is relatively low. At time 12 an attempt to start the compressor is initiated, i.e. the compressor state 9 is moved to ‘on’. Since the surface temperature 11 of the condenser is relatively low, the attempt to start the compressor is successful. After a while, at time 13, the compressor is once again switched ‘off’. At time 14 the surface temperature 11 of the condenser starts to increase, probably due to increasing ambient temperature.

The dotted line 15 indicates that some time has been allowed to lapse. During the lapsed time an attempted start-up of the compressor has been initiated. At time 16 it is determined that an attempt to start the compressor was unsuccessful. Since the surface temperature 11 of the condenser is relatively high, this is probably the cause. In one embodiment this may be established by measuring or monitoring the surface temperature 11 of the condenser or another temperature in the vicinity of the condenser. As a consequence the compressor state 9 is moved from ‘on’ to ‘off’. Shortly following that, at time 17, the condenser fan state 10 is moved from ‘off’ to ‘on’, i.e. the condenser fan is switched ‘on’ in order to provide cooling for the surface of the condenser. This has the effect that the surface temperature 11 of the condenser immediately starts to decrease. After a while, at time 18, another attempt to start the compressor is initiated, i.e. the compressor state 9 is once again switched to ‘on’. This time the attempt to start the compressor is successful because the surface temperature 11 of the condenser has been allowed to decrease sufficiently. At time 19 the compressor as well as the condenser fan is switched ‘off’. Thus, in this situation the condenser fan is allowed to run during all the time the compressor is ‘on’ in order to ensure that the surface temperature 11 of the condenser remains sufficiently low or even decreases further after the compressor has been started.

In case the surface temperature 11 of the condenser (or another temperature in the vicinity of the condenser) is measured or monitored, it may be determined that the condenser fan needs to be switched to the ‘on’ state while the compressor is ‘on’, even though the start-up of the compressor was successful. In this case the condenser will be switched ‘on’, and it will preferably remain in the ‘on’ state until the compressor is switched ‘off’. However, the condenser fan is still only switched ‘on’ when necessary.

FIG. 3 is a flow chart illustrating a method according to an embodiment of the invention. Initially a start-up sequence is started at step 20. At step 21 it is attempted to start the compressor. At step 22 it is investigated whether or not the attempt to start the compressor was successful. If this is the case, the process continues to step 23 where the start-up sequence is ended. If it is determined at step 22 that the attempt to start the compressor was unsuccessful, this may be due to the pressure at the condenser side of the compressor is too high due to a high surface temperature of the condenser. Consequently, the process continues to step 24 where a condenser fan is switched on. Thereby cooling is provided for the condenser in order to lower the temperature of the condenser, thereby lowering the pressure of the refrigerant present at the condenser side of the compressor. Subsequently the process continues to step 25 where the lapse of a predefined time interval, in this case 1 min., is awaited in order to allow the temperature of the condenser to become sufficiently low. Subsequently the process is returned to step 21 for a new attempt to start the compressor.

According to the method illustrated in FIG. 3 the condenser fan is only switched on if it is necessary to cool the condenser in order to ensure a successful start-up of the compressor. If the condenser is already sufficiently cold to ensure a successful start-up of the compressor, there is no need to cool it further, and the energy which would otherwise be used for driving the fan is therefore saved according to the invention. On the other hand, if the temperature of the condenser is too high to allow a successful start-up of the compressor, it is ensured that cooling is provided for the condenser. Thereby reliable start-up of the compressor is ensured with as low energy consumption as possible. This is very advantageous.

While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present invention. 

1. A method for starting a compressor in a refrigeration system, the refrigeration system further comprising a condenser and an evaporator, said compressor, condenser and evaporator being interconnected in a circuit in which a refrigerant is allowed to flow, the method comprising the steps of: attempting start-up of the compressor, thereby initiating a start-up sequence, determining whether the attempted start-up of the compressor was successful, in the case that it is determined that the attempted start-up of the compressor was unsuccessful, starting a condenser cooling means, thereby providing cooling for the condenser, and waiting for a predefined time interval, repeating the above steps until it is determined that the attempted start-up of the compressor was successful, and ending the start-up sequence.
 2. The method according to claim 1, wherein the predefined time is within the time interval 10 s to 300 s.
 3. The method according to claim 1, wherein the step of starting a condenser cooling means comprises supplying power to a fan positioned in the vicinity of the condenser, thereby causing the fan to rotate.
 4. The method according to claim 1, further comprising the step of stopping the condenser cooling means after the predefined time has elapsed.
 5. The method according to claim 1, further comprising the steps of monitoring a temperature in the vicinity of the condenser, and starting the condenser cooling means in case said temperature exceeds a threshold temperature.
 6. A control unit for controlling start-up of a compressor in a refrigeration system, the refrigeration system further comprising a condenser and an evaporator, said compressor, condenser and evaporator being interconnected in a circuit in which a refrigerant is allowed to flow, the control unit comprising means for starting a condenser cooling means in case it is determined that an attempted start-up of the compressor was unsuccessful.
 7. The control unit according to claim 6, wherein the condenser cooling means comprises a fan positioned in the vicinity of the condenser, and wherein the means for starting the condenser cooling means comprises means for supplying power to the fan, thereby causing it to rotate.
 8. The control unit according to claim 6, further comprising means for monitoring a temperature in the vicinity of the condenser, and wherein the means for starting the condenser cooling means is adapted to start the condenser cooling means in case said temperature exceeds a threshold temperature.
 9. A starter assembly for starting a compressor in a refrigeration system, the starter assembly comprising a control unit according to claim
 6. 10. The starter assembly according to claim 9, wherein the starter assembly and/or the compressor is/are powered by means of a battery. 